Usually when I pick the slow free shipping they seem to sit on it for at least a day or two just to make sure I know what slow free shipping is. But there was that stretch last year where everything was next day.
Last week, I ordered a portable radio, three pairs of rechargeable C-batteries, and a patch cable. It was a choice of free two-day shipping on Prime or add $20 ($4 * 5 items) for one-day. I went with free, but it arrived a day early anyway.